The present invention relates generally to the field of heat exchanger tube supports, and in particular to a new and useful U-bend support system for positioning and restraining the U-bends of water tubes within a nuclear steam generator against flow-induced vibration.
In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine. Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 46, 48 and 50 of Steam/Its Generation and Use, 41st Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., ©2005, the text of which is hereby incorporated by reference as though fully set forth herein.
Nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. In the U-bend region of a nuclear steam generator, a large flow of steam and water mixture passes upwards through the tube array, in a general direction which locally is normal to the axis of the individual U-bend tubes. This large two phase flow is able to cause excitation of the U-bend tubes via the turbulent and other flow forces imparted by the flow. As a result, the tubes tend to vibrate in both the out-of-plane and in-plane directions relative to the U-bend plane. Typically this restraint function is provided by an array of flat U-bend support bars. While such flat bars provide positive restraint in the U-bend out-of-plane direction, they provide restraint only by friction in the in-plane direction.
One known type of nuclear steam generator U-bend support assembly, depicted in FIG. 1, and in greater detail in FIG. 2, is manufactured by Babcock & Wilcox Canada Ltd. FIG. 1 shows a nuclear steam generator 80 having a plurality of U-bend tubes 102, referred to as a tube bundle, which are fixed at their ends to a heavy tubesheet 90. The U-bend tubes 102 are arranged in layers or columns. Each layer or column incorporates a set of tubes of successively larger radius, which are nested, from innermost tube to outermost tube, to create the layer or column of tubes in the particular plane. The tubes are further arranged in rows, with each row containing all tubes of a particular U-bend radius. For purposes of illustration, however, FIG. 1 shows only a limited number of U-bend tubes 102, and FIG. 2 shows only the outermost tubes of the center U-bend layers. The straight leg portions of the U-bend tubes 102 are supported at several locations by vertically spaced apart tube support plates 120 as shown in FIG. 1.
The U-bend portions 103 of tubes 102 extend beyond the uppermost tube support lattice (or plate) 124 and sweep through 180 degrees of arc. The relatively long U-bend region 103 of each U-tube 102 requires supports to keep them in position and to restrain against flow-induced vibration (FIV) excitation due to the very large upward flow of two-phase steam/water mixture.
As shown in FIG. 1, and in greater detail in FIG. 2, the U-bend tubes 102 are positioned and restrained in the U-bend region 103 of U-bend tubes 102 by a U-bend support assembly 100, which includes a number of U-bend support bar arrays 180. Each U-bend support bar array 180 is comprised of flat U-bend support bars 160, which are positioned in sets between layers of tubes within the U-bend region of the steam generator.
As shown in FIG. 2, the flat U-bend support bars 160 fan out from the center of the U-bend such that individual bar sets are assembled into a U-bend support bar array 180, or “fan” bar array, in which the inner ends of the individual bars are interconnected to collector bar 114 by a mechanical or welded joint 190. U-bend support bar array 180 is referred to as a “half-fan” array, since collector bar 114 covers only half the U-bend region (i.e. either the cold leg or the hot leg) of tubes in a particular plane.
Each U-bend support bar array 180 incorporates about 4 to 12 of the flat U-bend support bars 160. The flat U-bend support bars 160 are positioned so as to provide support to the U-bend tubes 102 at certain points along the arc of each U-bend tube in the array. The angular separation of the flat U-bend support bars 160 depends upon the U-bend size and flow conditions; the flat U-bend support bars 160 are located to minimize unsupported tube lengths. The individual flat U-bend support bars 160 are typically made of stainless steel, and are about 1″ to 1.5″ wide and about 0.1″ to 0.2″ thick. A U-bend support assembly 100 may incorporate between about 100 to about 200 of the fan-shaped U-bend support bar arrays 180, with one such array located between each plane of U-bend tubes.
The outer ends of the flat U-bend support bars 160 are collected, restrained and supported by arch bar support structures, which extend in the out-of-plane direction, perpendicular to the columns or layers of U-bend tubes 102. Each arch bar structure is made up of arch bars 170 and clamping bars 175. Each arch bar 170 is a single continuous piece. The clamping bars 175 are segmented and affix the J-tabs 176 and the upper ends of the flat U-bend support bars 160 to arch bars 170. Each arch bar support structure positions the flat U-bend support bars 160 of a U-bend support bar array 180, carrying the weight of the bars and redistributing the weight of the U-bend support assembly 100 back to the peripheral U-bend tubes via J-tabs 176. Tie tubes 150, arranged horizontally above arch bars 170 and interconnecting the arch bar support structures at selected locations, restrain the fan bar arrays in position on the U-bends.
The U-bend support bar arrays 180 position the planes of U-bend tubes 102 in space, and most importantly, restrain the individual U-bend tubes against flow induced vibration. Restraint against out-of-plane motion is provided by the physical presence of the flat U-bend support bars 160, which are situated immediately adjacent to the U-bend tubes 102. The bar-to-tube clearance is purposely quite small, with individual bar-to-tube diametral clearances varying from about 0 to 0.010″ or more. The flat U-bend support bars 160, with their small bar-to-tube clearances, thus prevent significant motion of the tubes in the out-of-plane direction 140. In the in-plane direction 130, however, the U-bend tubes 102 are not positively restrained, but instead depend solely upon friction between the U-bend tubes 102 and the flat U-bend support bars 160 to restrict and dampen the flow induced motion of the tubes in their in-plane direction. Depending on the design details and flow conditions, the effect of the friction in providing in-plane restraint may not be fully adequate in providing effective in-plane restraint.
U.S. Pat. No. 6,772,832, which is assigned to the assignee of the present invention, discloses a corrective retrofit tube support structure having rows of concave pockets located on diagonally opposite surfaces of the bar.